1. A geometric figure with three sides.
2. A three-sided handheld instrument, usually classified as percussion, played with a mallet.
3. A female genital piercing that circles the clitoris- not to be confused with hood, which pierces the skin over it.
"When I was in second grade, my teacher told me I should ding the triangle instead of singing in the choir."
Someone you throw paper airplanes at, perform as a Tourette's child for, compliment on her dress and say you have one just like it (if you are male), and generally confuse and irritate. The ultimate goal is to either make the substitute break down laughing or reveal his/her shocking brain-dead-ness by convincing him/her that you really do talk like that.
"Wow, that sub we had in history was wearing a belt and suspenders."